The invention relates to an apparatus for coating films in a vacuum with metallic or nonmetallic materials. A unit prepares the film for the coating process, and pulley rolls provide close contact between the film and a chilled coating roll during a coating by a source excited by thermalization.
The coating of films, which as a rule have been limited resistance to heat, is performed in common processes with coating sources which are operated preferably by thermalizing the coating material. The technical problem therefore arises that, for the formation of a sufficient coating thickness on the film, the film itself must be exposed to the coating source for a certain minimum amount of time, and hence to the heat source. In this case, however, the film undergoes considerable heating and if the coating time is too long or the coating rate too high it can be thermally destroyed.
Usually the film to be coated is unwound from a parent roll on which the uncoated film is stocked, and brought over various pulley rolls within a coating chamber into contact with a cooled coating roll which keeps the film at the lowest possible temperature while it is being coated.
The cooling effect of the coating roll, however, is very limited, inasmuch as the thermal transfer coefficient between the film and the coating roll is determined by the mechanical contact between the film and the roll. Since the roughness of the roll and of the film permits only an interrupted surface contact, the heat transfer coefficient is about 30 to 50 W/m.sup.2 k (Watts/m.sup.2 0,K) which is too low to cool the film during the coating process on account of the high temperatures.
Further studies of the heat flow from the film to the coating roll have shown that, during the coating process, a water vapor cushion forms between the film and the coating roll, which is due to the desorption of the water adsorbed onto the film surface and fills up the microscopic voids that are formed by the roughness between the film and the coating roll. See, in this connection, the article, "Thermal Limitations in Roll Coating Processes," by W. Schwartz and W. Wagner, which appeared in `Society of Vacuum Coaters,` 1985. The vapor cushion thus forming provides for a thermal transfer sufficient for the coating of the film, with a heat transfer coefficient between 300 and 500 W/m.sup.2 k.
Heat transfer coefficients of this magnitude require films which have an initially present amount of water adsorbed onto their surface. In some coating processes, for example in two-sided coating, in which during the second coating the back of the film has already been coated, no more moisture is adsorbed onto the film surface due, of course, to the first coating process. Therefore no vapor cushion can form between the film and the coating roll, so that thermal damage can be done to the film.